1. Field of the Invention
The present invention relates to an optical fiber containing fluorescent doping substances, adapted to carry out the amplification of an optical transmission signal sent thereto and to eliminate radiations having an undesired wavelength, produced at the inside thereof by spontaneous emission.
2. Description of the Related Art
It is known that optical fibres having rare-earth doped cores may be used in optical amplifiers. For example, erbium doped cores pumped with a suitable wavelength pump source (for example 532, 670, 807, 980, or 1490 nm) can be used as a travelling wave amplifier for optical signals in the 1550 nm telecommunications wavelength region.
These fibres in fact can be supplied by a light source having a particular wavelength which is capable of bringing the doping substance atoms to an excited energetic state, or pumping band, from which the atoms spontaneously decay in a very short time to a laser emission state, in which state they stand for a relatively longer time.
When a fiber having a high number of atoms in excited state in the emission level is crossed by a luminous signal with a wavelength corresponding to such emission laser state, the signal causes the transition of the excited atoms to a lower level with a light emission having the same wavelength as the signal; therefore a fiber of this kind can be used to achieve an optical signal amplification.
Starting from the excited state the atom decay can occur also spontaneously, which gives rise to a random emission constituting a "background noise" overlapping the stimulated emission, corresponding to the amplified signal.
The light emission generated by the introduction of luminous pumping energy into the "doped" or active fiber can take place at several wavelengths, typical of the doping substance, so as to give origin to a fluorescence spectrum of the fiber.
In order to achieve the maximum signal amplification by means of a fiber of the above type, together with a high signal-to-noise ratio, in optical telecommunications it is normally used a signal generated by a laser emitter having a wavelength corresponding to a maximum of the fluorescence spectrum curve of the fiber incorporating the doping substance used.
In particular, when optical telecommunication signal amplifications are concerned, the use of "active" fibres having a core doped with erbium ions (Er.sup.3+) is convenient.
However, the spectral gain profile of an erbium doped core in an amplifier of the type above described is characterized by two gain bands. One narrow gain band is centred around 1530 nm and a second broader but lower level gain band is centred around 1550 nm.
The peak wavelengths of the gain bands and their spectral widths are dependent on the host glass composition of the core. For example silica cores doped with erbium and germania have the higher gain band peak wavelength at 1536 nm and silica cores doped with erbium and alumina have the higher gain band peak wavelength at 1532 nm.
In both cases, the higher gain band has a "3 dB linewidth" of about 3 to 4 nm, and the lower level gain band, depending on the host glass composition is broader with a "3dB linewidth" of about 30 nm. The former gain band exhibits greater gain than the latter but requires the signal to be amplified to have a very stable, tightly specified centre wavelength.
This dictates the use, as the transmission signal source, of a laser emitter operating to a well defined wavelength with a limited tolerance, because signals exceeding such tolerance limits would not be properly amplified, while at the same time a strong spontaneous emission would occur at this peak wavelength which would give rise to a background noise capable of greatly impairing the transmission quality.
Laser emitters having the above features, that is operating at the erbium emission peak, are however of difficult and expensive production, whereas the common industrial production offers laser emitters such as semiconductor lasers (In, Ga, As), exhibiting several features making them suitable for use in telecommunications but having a rather large tolerance as regards the emission wavelength and therefore only a reduced number of laser emitters of this kind has an emission at the above peak wavelength.
While in some applications, such as for example submarine telecommunications, the choice can be accepted of using transmission signal emitters operating at a well defined wavelength value, for example obtained through an accurate selection from lasers of commercial quality so as to use only those having an emission strictly close to the laser emission peak of the amplifier fiber, this procedure is not acceptable from an economical point of view when other kinds of lines are concerned, such as for example municipal communication lines where it is of great importance to limit the installation costs.
For example, an erbium-doped fiber adapted to allow the laser emission has an emission peak at approximately 1536 nm and over a range of a .+-.5 nm from said emission value it has a high intensity and can be used to amplify a signal in the same wavelength range; however, commercially available semiconductor lasers to be used for transmission are usually made with emission wavelength values in the range of 1520 to 1570 nm.
As a result, a great number of commercially available lasers are at the outside of the range adapted for the erbium-based amplification and therefore cannot be employed for generating telecommunication signals in lines provided with erbium-based amplifiers of the above type.
On the other hand it is known that erbium-doped fibres have the above discussed second gain band in the emission spectrum with a relatively high and substantially constant intensity in a wavelength range contiguous to the above described narrow gain peak, wide enough to include therein the emission range of the above mentioned commercially available lasers.
However, in an optical fiber of this type a signal having wavelength in the second gain band would be amplified in a reduced measure, whereas spontaneous transitions from the laser emission state in the fiber mainly take place with the emission at the wavelength of the narrow gain band at 1536 nm, thus generating a "background noise" which will be further amplified through the active fiber length and will overlap the useful signal.
It may be envisaged to carry out the filtering of the luminous emission constituting the "noise" at the end of the amplifier fiber, sending to the line the only wavelength of the transmission signal, for the purpose providing a suitable filter at the end of the active fiber.
However the presence of a spontaneous emission in the fiber mainly at the wavelength of the fiber maximum amplification would subtract pumping energy to the transmission signal amplification having a different wavelength, thus making the fiber substantially inactive as regards the amplification of the signal itself.
The problem arises therefore of providing an active optical fiber to be employed in optical amplifiers which is adapted to be used together with commercially available laser emitters for the emission of the transmission signal without important qualitative restrictions being imposed to said laser emitters.